1. Field of the Invention
A circuit arrangement is provided for determining the output power of switched-mode power supplies so that the output power can be determined independent of the frequency and the input voltage.
2. Description of the Related Art
Switched-mode power supplies are characterized by the fact that they deliver a constantly corrected d.c. voltage that is optimally independent of the load. The power output can encompass a broad range. For example, for television devices, the output power range of a switched-mode power supply can range from 5 W to 150 W. For a lower delivered power, a low switching frequency is often set in order to minimize switching losses, or circuit components of the television device are switched in or out. To minimize these losses, the instantaneous power of the switched-mode power supply must be measured at the primary side and appropriately conditioned for controlling the frequency or controlling circuit components. A simple way to determine the power is to measure the primary current in the switching transistor using a measuring resistor. The power is proportional to the square of the current. Since the level of this current also depends heavily on the frequency and the input voltage, this manner of power determination is highly inaccurate.
German Patent Document DE 3 129 021 A1 teaches a circuit arrangement for measuring the through power in d.c. transformers, however, this arrangement is only suitable for transformers that are timed with a constant frequency.